Stacked Wafer-Bonded Gradient Index Silicon Optics with Integral Antireflection Treatment

A wide range of applications in submillimeter and millimeter wavelength astronomy, on the ground and in space, would benefit from silicon optics with broadband antireflection treatment. Silicon's high refractive index and low loss make it an ideal optical material at these wavelengths. It is even possible to use silicon for ambient temperature vacuum windows. Antireflection treatment of silicon optics is essential, however, and has proven a major challenge for the 150 mm to 1000 mm diameter optics required for current and future applications. Moreover, multilayer antireflection treatments are necessary for wide spectral bandwidths, with wider bandwidths requiring more layers. It is difficult to find low loss dielectrics with the correct refractive index and other properties to match silicon well, especially if more than one layer is required. Textured surfaces are an attractive alternative to dielectric antireflection coatings. This approach has been most successfully applied for silicon lenses in the ACTpol experiment at mm wavelengths using a dicing saw to cut a two-layer AR structure that gave a < -23 dB reflection intensity over a spectral bandwidth of 1.3:1. The same group has an unpublished prototype of a 5-layer structure. Others have used the dicing saw approach to produce smooth-sided pyramids. Laser machining has also been applied to produce circular holes, sharp cones, concentric circular grooves, and pyramids. Extending to submm wavelengths and increasing the bandwidth requires finer and more accurate features than conventional machining, and perhaps laser machining, can produce. Depth control is also a challenge for laser machining. An alternative is to use a photolithographic fabrication process to etch features onto the silicon's surface. Deep reactive ion etching (DRIE) is a mature micromachining technique that can create deep features (up to 30:1 in aspect ratio) in arbitrary patterns, and it was previously used in demonstrations of flat single-layer coatings at THz frequencies. We have extended this approach on flat surfaces using a unique, multi-depth DRIE technique. We have published excellent results on a two-layer, 1.6:1 bandwidth AR structure patterned using multi-depth DRIE. This is the first *optical* demonstration of an *etched*, two-layer structure of which we are aware. We are fabricating a four-layer structure with an expected 4:1 bandwidth, incorporating wafer-bonding to assemble the AR structure, and our approach is scalable to a bandwidth of 6:1 with a 7-layer structure. Here, we propose to build on this work to realize powered optics with such integral AR structures. To circumvent applying the above process to a curved surface, or to slumping a flat structure onto a curved surface, we will *construct* a silicon optic by stacking flat patterned wafers. The starting point is a multilayer optical design incorporating both an axial gradient in the refractive index for antireflection and a radial index gradient for focusing. For each optical layer, we will use a radially varying hole pattern to achieve the required effective index of refraction. Using our multi-depth DRIE process, we will fabricate multiple layers of the optical structure on each individual flat wafer. Using our demonstrated wafer-bonding technique, we will stack several of these wafers and bond them together to produce the completed optic. Our experience so far with flat AR structures provides positive evidence this approach will work. The estimated costs for "production mode" are reasonable. We are in the midst of producing a first-generation gradient index optic. We will add to this proof-of-principle optic design AR structures and then extend the approach to larger bandwidths, building on the demonstrated flat, wide-bandwidth AR structures. The work will culminate with a demonstration optic having a 6:1 bandwidth, 100 mm in diameter. This work will advance the TRL from 2 to 4.

The Astrophysics Research and Analysis program (APRA) supports suborbital and suborbital-class investigations, development of detectors and supporting technology, laboratory astrophysics, and limited ground based observing. Basic research proposals in these areas are solicited for investigations that are relevant to NASA's programs in astronomy and astrophysics, including the entire range of photons, gravitational waves, and particle astrophysics. The emphasis of this solicitation is on technologies and investigations that advance NASA astrophysics missions and goals.

This is early/mid-stage (TRL 1) — the most realistic path in is NASA SBIR/STTR, which funds small businesses and research institutions to develop technology aligned with NASA's needs (equity-free, phased funding). Check whether a current SBIR/STTR solicitation topic overlaps with this project's technology area, or contact the project directly (above) to ask.

None of these are guaranteed paths for this specific project — TechPort itself doesn't have an "apply" button. Reaching out to the contact(s) above with a specific question is usually the fastest way to find out what's actually open.